1. Field of the Invention
The invention relates to the field of pumps that employ a fluid under pressure for motive power, for example using steam pressure to pump liquid condensate for removal or recovery of condensate in a steam system, heat exchanger or other pressurized apparatus. In particular the invention concerns an improved pump employing gas under pressure as the fluid displacement means and having a venting valve that opens during the exhaust phase of a pumping cycle to effectively enlarge the exhaust orifice.
2. Prior Art
Pumps powered by gas pressure, especially steam pressure, have a number of benefits for pumping liquids. Such pumps can operate under various conditions of pressure or vacuum, and do not require seals or packings as do pumps powered by rotary machines or having pistons or centrifugal impellers. Pressure driven pumps consume a minimal amount of power and generally provide a durable and cost effective solution to pumping needs in various situations.
A typical pump driven by gas pressure comprises a tank having a liquid inlet and a liquid outlet near the bottom of the tank, with an inlet check valve and an outlet check valve permitting flow only in the pumping direction. The tank also has a gas inlet and a gas exhaust outlet located higher on the tank, above the maximum liquid level. The gas inlet and gas outlet have valves that are operated reciprocally, such that the gas or pressure inlet is open when the gas outlet or exhaust is closed, and vice versa, as a function of the level of liquid in the pump tank. For example, the gas inlet valve and gas outlet valve can be coupled to a float mechanism. Alternatively, the liquid level in the tank can be sensed by electrical level sensors that produce a signal for triggering the gas or pressure inlet/outlet valves to reverse positions. The operation requires a certain hysteresis, with the gas inlet opening and exhaust closing when the fluid level reaches a high threshold level, and remaining in that position until reversing when the fluid level drops below a low threshold. The difference between the thresholds, which can be sensed in a variety of ways, defines the stroke of the pump.
One arrangement in which the liquid level is sensed using a float and the valves are operated mechanically, involves a snap action linkage that simultaneously opens the gas inlet and closes the gas outlet, or closes the gas inlet and opens the gas outlet, at the two thresholds. Examples of such snap action float mechanisms and pumps are disclosed in U.S. Pat. Nos. 5,230,361--Carr et al.; 5,366,349--Ilg; 5,141,405--Francart, Jr.; 1,699,464--Dutcher, etc.
The pump has a cycle including a liquid filling phase and a liquid discharge phase. During the liquid filling phase the gas inlet is closed, the gas outlet is open, and the liquid, which can be water or some other liquid, flows at a relatively low pressure through the liquid inlet check valve to fill the tank. This filling flow can be powered by gravity or another form of low pressure flow. The liquid outlet check valve remains closed because the pressure of the liquid in the tank is relatively low. Tank pressure is low because the gas exhaust valve is open, and the flow line downstream of the outlet check valve may be pressurized as well, either of which keeps the outlet check valve closed. The exhaust valve may vent into the atmosphere, or it may vent into a closed conduit or vessel at a pressure less than the liquid inlet head.
As the float rises in the tank with the level of liquid, the float mechanism reaches a crossover point and toggles the gas valves to open the gas inlet and close the gas outlet, switching from the liquid filling phase of the cycle to the liquid discharge phase. Gas under pressure, such as steam, pressurizes the tank through the gas inlet valve, the gas outlet valve now being closed. Gas pressure builds in the tank, reverse biases the liquid inlet check valve, and forward biases the liquid outlet check valve. The liquid in the tank is forced by gas pressure through the liquid outlet check valve and downstream of the pump, at the pressure of the steam or other gas. When the float drops past a low crossover point, the gas inlet valve closes and the gas outlet valve opens, venting the pressure in the tank and permitting the cycle to repeat.
In this manner the tank alternately fills with low pressure liquid and discharges at higher pressure through the liquid outlet. The pump is useful for returning or inserting liquid such as water into a pressurized system using the pressure in the system as the motive pumping force. This is particularly useful in connection with steam power and heat exchange systems. However, all that is needed is a pressure differential. Thus the pump is useful in closed loop arrangement in which one or more of the inlet liquid feed to the tank, the gas exhaust from the tank and the outlet, are at elevated pressure as compared to ambient.
Although a pressure pump as described is durable and useful, there are certain limitations inherent in its structure, resulting in limitations on the flow or pumping capacity of the pump. Inasmuch as liquid filling typically is accomplished at low differential pressure (e.g., by gravity), the liquid fill rate can be slow. Moreover, when switching from the pressurized pump-out phase to the vented exhaust and filling stage, time is required to permit the gas pressure in the tank to vent before low pressure liquid can begin to fill the tank through the liquid inlet check valve. The time taken to reduce the internal tank pressure to a lower pressure than the inlet line depends on several factors including the extent to which the tank was pressurized and the internal diameter and back pressure of the gas exhaust valve and conduit. The need to vent and reduce tank pressure to shift from positive to negative pressure differentials between the tank and the liquid inlet (to open the inlet check valve and allow an in-flow) and between the tank and the liquid outlet (to close the outlet check valve), respectively, provide an inherent cycling delay and a corresponding limitation on the flow rate of the pump.
Where the liquid being pumped is water and the gas pressure is provided by pressurized steam, limitations on pumping and flow capacity are aggravated because in the pumping phase the pressurized steam heats the walls of the tank, which can reach a temperature higher than the boiling point of water at the lower pressure characteristic of the filling phase. In that case, water flowing initially into the tank through the inlet check valve (as well as residual water already in the tank) boils and generates additional steam and pressure that must be vented through the exhaust valve. Inlet water that is already near boiling temperature is of course more prone to boil when it contacts the steam-heated walls of the tank. The flow restriction caused by the exhaust valve limits the extent to which the motive steam pressure and the boiling water steam pressure can vent.
It would be possible to provide a very large exhaust orifice in order to vent the tank quickly when switching from the pressurized pumping phase to the venting fill phase. However, the exhaust valve must be forced open against the pressure in the tank at the point of the switchover from pumping to filling, for example by the force generated by the spring of a snapover float mechanism.
Where the gas inlet and outlet valves are linked mechanically, the device that opens the gas inlet valve and closes the gas outlet valve is opposed by differential pressure between the pressure source and the tank for opening the inlet to commence a pumping phase, and between the tank and the vent for opening the outlet valve to commence a filling phase. In a pump vented to the atmosphere the pressure differential in each case is substantially equal to the difference between the gas supply pressure and ambient pressure. Or in a closed system the differential is between the pressures of the gas supply and the vent line. If one chooses to enlarge the orifice size of the exhaust valve to speed or improve venting, the surface area of the exhaust valve body is increased. As a result, a correspondingly larger force is needed to open the exhaust valve against the pressure differential, because the same force per unit of area is applied to a larger area. It is not desirable to add heavier springs or other expensive mechanical features to the mechanism that operates the respective valves. Likewise, larger valves are generally more expensive and technically demanding than smaller ones, particularly for high pressure applications.
What is needed is a means to reduce the flow restriction at the exhaust of a pump, that is to enlarge the exhaust orifice, without the drawbacks of a large valve including the need to obtain added mechanical opening force in the valve operating mechanism. Further, the exhaust valve structure should deal with the problem of venting steam generated by water boiling upon flowing into a superheated tank or upon a drop in tank pressure, such that the steam does not substantially slow the venting of pressure or the inflow of water.